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Cortical sources of Vernier acuity in the human visual system: An EEG-source imaging study
Get PDFVernier acuity determines the relative position of
visual features with a precision better than the
sampling resolution of cone receptors in the retina.
Because Vernier displacement is thought to be
mediated by orientation-tuned mechanisms, Vernier
acuity is presumed to be processed in striate visual
cortex (V1). However, there is considerable evidence
suggesting that Vernier acuity is dependent not only
on structures in V1 but also on processing in
extrastriate cortical regions. Here we used functional
magnetic resonance imaging–informed
electroencephalogram source imaging to localize the
cortical sources of Vernier acuity in observers with
normal vision. We measured suprathreshold and nearthreshold
responses to Vernier onset/offset stimuli at
different stages of the visual cortical hierarchy,
including V1, hV4, lateral occipital cortex (LOC), and
middle temporal cortex (hMTþ). These responses were
compared with responses to grating on/off stimuli, as
well as to stimuli that control for lateral motion in the
Vernier task. Our results show that all visual cortical
regions of interest (ROIs) responded to both
suprathreshold Vernier and grating stimuli. However,
thresholds for Vernier displacement (Vernier acuity)
were lowest in V1 and LOC compared with hV4 and
hMTþ, whereas all visual ROIs had identical thresholds
for spatial frequency (grating acuity) and for relative
motion. The cortical selectivity of sensitivity to Vernier
displacement provides strong evidence that LOC, in
addition to V1, is involved in Vernier acuity processing.
The robust activation of LOC might be related to the
sensitivity to the relative position of features, which is
common to Vernier displacement and to some kinds of
texture segmentation.
Copyright 2017 The Authors1011Nsciescopu
Ensemble representations reveal distinct neural coding of visual working memory
Get PDF© 2019, The Author(s).We characterized the population-level neural coding of ensemble representations in visual working memory from human electroencephalography. Ensemble representations provide a unique opportunity to investigate structured representations of working memory because the visual system encodes high-order summary statistics as well as noisy sensory inputs in a hierarchical manner. Here, we consistently observe stable coding of simple features as well as the ensemble mean in frontocentral electrodes, which even correlated with behavioral indices of the ensemble across individuals. In occipitoparietal electrodes, however, we find that remembered features are dynamically coded over time, whereas neural coding of the ensemble mean is absent in the old/new judgment task. In contrast, both dynamic and stable coding are found in the continuous estimation task. Our findings suggest that the prefrontal cortex holds behaviorally relevant abstract representations while visual representations in posterior and visual areas are modulated by the task demands.11Nsciescopu
Attention to multiple objects facilitates their integration in prefrontal and parietal cortex
No full textSelective attention is known to interact with perceptual organization. In visual scenes, individual objects that are distinct and discriminablemayoccur on their own, or in groups such as a stack of books. The main objective of this study is to probe the neural interaction that occurs between individual objects when attention is directed toward one or more objects. Here we record steady-state visual evoked potentials via electrocorticography to directly assess the responses to individual stimuli and to their interaction. When human participants attend to two adjacent stimuli, prefrontal and parietal cortex shows a selective enhancement of only the neural interaction between stimuli, but not the responses to individual stimuli.Whenonly one stimulus is attended, the neural response to that stimulus is selectively enhanced in prefrontal and parietal cortex. In contrast, early visual areas generally manifest responses to individual stimuli and to their interaction regardless of attentional task, although a subset of the responses is modulated similarly to prefrontal and parietal cortex. Thus, the neural representation of the visual scene as one progresses up the cortical hierarchy becomes more highly task-specific and represents either individual stimuli or their interaction, depending on the behavioral goal. Attention to multiple objects facilitates an integration of objects akin to perceptual grouping. © 2017 the authors1111Nsciescopu
Induced astigmatism biases the orientation information represented in multivariate electroencephalogram activities
No full text© 2021 The Authors. Human Brain Mapping published by Wiley Periodicals LLC.A small physical change in the eye influences the entire neural information process along the visual pathway, causing perceptual errors and behavioral changes. Astigmatism, a refractive error in which visual images do not evenly focus on the retina, modulates visual perception, and the accompanying neural processes in the brain. However, studies on the neural representation of visual stimuli in astigmatism are scarce. We investigated the relationship between retinal input distortions and neural bias in astigmatism and how modulated neural information causes a perceptual error. We induced astigmatism by placing a cylindrical lens on the dominant eye of human participants, while they reported the orientations of the presented Gabor patches. The simultaneously recorded electroencephalogram activity revealed that stimulus orientation information estimated from the multivariate electroencephalogram activity was biased away from the neural representation of the astigmatic axis and predictive of behavioral bias. The representational neural dynamics underlying the perceptual error revealed the temporal state transition; it was transiently dynamic and unstable (approximately 350 ms from stimulus onset) that soon stabilized. The biased stimulus orientation information represented by the spatially distributed electroencephalogram activity mediated the distorted retinal images and biased orientation perception in induced astigmatism.11Nsciescopu
Motion direction representation in multivariate electroencephalography activity for smooth pursuit eye movements
Get PDFVisually-guided smooth pursuit eye movements are composed of initial open-loop and later steady-state periods.
Feedforward sensory information dominates the motor behavior during the open-loop pursuit, and a more
complex feedback loop regulates the steady-state pursuit. To understand the neural representations of motion
direction during open-loop and steady-state smooth pursuits, we recorded electroencephalography (EEG) responses
from human observers while they tracked random-dot kinematograms as pursuit targets. We estimated
population direction tuning curves from multivariate EEG activity using an inverted encoding model. We found
significant direction tuning curves as early as about 60 ms from stimulus onset. Direction tuning responses were
generalized to later times during the open-loop smooth pursuit, but they became more dynamic during the later
steady-state pursuit. The encoding quality of retinal motion direction information estimated from the early direction
tuning curves was predictive of trial-by-trial variation in initial pursuit directions. These results suggest
that the movement directions of open-loop smooth pursuit are guided by the representation of the retinal motion
present in the multivariate EEG activity. © 2019 Elsevier Inc. All rights reserved.11Nsci
The representational dynamics of sequential perceptual averaging
No full textIt is clear that humans can extract statistical information from streams of visual input, yet how our brain processes sequential images into the abstract representation of the mean feature value remains poorly explored. Using multivariate pattern analyses of electroencephalgraphy recorded while human observers viewed the sequentially presented ten Gabors of different orientations to estimate their mean orientation at the end, we investigated sequential averaging mechanism by tracking the quality of individual and mean orientation as a function of sequential position. Critically, we varied the sequential variance of Gabor orientations to understand the neural basis of perceptual mean errors occurring during sequential averaging task. We found that the mean-orientation representation emerged at specific delays from each sequential stimulus onset and became increasingly accurate as additional Gabors were viewed. Especially in frontocentral electrodes, the neural representation of mean orientation improved more rapidly and to a greater degree in less volatile environment while individual orientation information was encoded precisely regardless of environmental volatility. The computational analysis of behavioral data also showed that perceptual mean errors arise from the cumulative construction of the mean orientation rather than the low-level encoding of individual stimulus orientation. Thus, our findings provide neural mechanisms to differentially accumulate increasingly abstract feature from a concrete piece of information across the cortical hierarchy depending on
49 environmental volatility.11Nsciescopu
Frontal-to-visual information flow explains predictive motion tracking
No full textPredictive tracking demonstrates our ability to maintain a line of vision on moving objects even when they temporarily disappear. Models of smooth pursuit eye movements posit that our brain achieves this ability by directly streamlining motor programming from continuously updated sensory motion information. To test this hypothesis, we obtained sensory motion representation from multivariate electroencephalogram activity while human participants covertly tracked a temporarily occluded moving stimulus with their eyes remaining stationary at the fixation point. The sensory motion representation of the occluded target evolves to its maximum strength at the expected timing of reappearance, suggesting a timely modulation of the internal model of the visual target. We further characterize the spatiotemporal dynamics of the task-relevant motion information by computing the phase gradients of slow oscillations. We discovered a predominant posterior-to-anterior phase gradient immediately after stimulus occlusion; however, at the expected timing of reappearance, the axis reverses the gradient, becoming anterior-to-posterior. The behavioral bias of smooth pursuit eye movements, which is a signature of the predictive process of the pursuit, was correlated with the posterior division of the gradient. These results suggest that the sensory motion area modulated by the prediction signal is involved in updating motor programming
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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